SPID#: 25 The objective of this research is to evaluate the toxicity and/or immunosuppressive effects of novel drug combinations with assessment of transport and metabolic disposition. In particular, effects of selected modulating agents on anti-HIV drugs will be studied in an attempt to prevent or alleviate bone marrow toxicity associated with chronic treatment with certain anti-HIV nucleoside analogs. Pharmacokinetics of anti-HIV drugs and modulation will be studied. Efficacy of selected drug combinations (anti-HIV and modulating agent) will eventually be assessed in SIV-infected rhesus monkeys.